| Cross-linked polyethylene(XLPE)distribution cables have been in large-scale service for more than 20 years in our country and they are entering the aging stage.There is a risk of centralized outbreak of insulation faults of distribution cables,and the safety of distribution network operation faces unprecedented challenges.The space span of a distribution cable is large and its laying environment is complex,the coexistence of multiple laying methods or the presence of local heat sources can easily lead to differences in insulation aging states along the cable line after long-term operation(the cable above is defined as the non-uniform aging cable).Due to the large scale of distribution cables,early decommissioning of cables with less serious overall aging or delayed service of cables nearing the end of life will cause greater economic loss or social impact.Thus,accurate diagnosis of the aging state of cables is of great significance for guiding the scientific operation and maintenance of distribution cables and ensuring the reliability of distribution network operation.For uniform aging cables,the existing typical diagnosis and evaluation methods are mainly based on destructive sampling techniques,which are not suitable for field application.For non-uniform aging cables,the traditional techniques,such as 0.1 Hz ultra-low frequency dielectric loss detection,polarization/depolarization current analysis,and frequency domain spectrum,can only obtain the average aging status of the cable,which cannot achieve a comprehensive evaluation of the aging status and residual life of the insulation along the cable line.This paper attempts to develop a non-destructive field test technology,which could solve the problem of aging diagnosis and residual life evaluation of uniform and non-uniform aging XLPE distribution cables.In view of this,the following research were carried out:Though broadband impedance spectroscopy(BIS)technology can identify the cable impedance discontinuities,it cannot identify the local aging segments and obtain their location.In this paper,a non-destructive method was proposed,in which the phase spectrum of the input impedance of a cable is used to be transformed by orthogonal transformation and then the imaginary part after transformation is extracted to construct a diagnostic function,then the aging segments and the concentration defects in the non-uniform aging cable could be identified and located according to the shapes of the "mutant peaks" in the diagnostic function.The unipolar "mutation peak"corresponds to the position of the beginning or end of the local aging section of the cable,and the location of the local aging segment can be determined according to the adjacent unipolar "mutation peak";the continuous heteropolar "mutation peak"corresponds to the location of the concentrated defect of the cable.Meanwhile,The polarity characteristics of the "mutation peak" could be used to assist in the qualitative analysis of the aging state of cable defects compared to that of cable segments on the left and right sides of them.Simulation and experimental results showed that the proposed method is also suitable for the identification and location of a cable aging segment with aging "gradient regions" at its both ends.In order to solve the problem that single BIS technology cannot quantitatively diagnose the aging status of cable insulation,a non-destructive method was proposed,which could calculate and obtain the wave speeds of cables at high frequencies(not less than 1 MHz)based on the resonant frequencies of input impedance spectrum.And the effect of uniform and segmented aging of cable insulation on the wave speed were studied and analyzed by simulation and experiment.For uniform aging cable,the more serious the cable aging state,the smaller the wave velocity,and the wave velocity is independent of the cable length.For the segmental aging cable,the increase of the segmental aging severity or the proportion of the length of the segmental aging area will lead to the decrease of the average wave speed in the cable,and the average wave speed of the cable is basically independent of the location of the segmental aging.Then a non-destructive method based on BIS technology was proposed to diagnose the insulation aging state of an uniform aging cable quantitatively by measuring the wave velocity of the cable.On this basis,the relationship between the wave velocity of the aging cable and the elongation at break(EAB)of the insulation layer was established,and it was set as a diagnostic criteria for cable insulation aging.Moreover,the effects of cables’ body temperatures and cross-section sizes on the wave velocities were further studied and analyzed,then a correction method for the aging diagnosis criteria of cables with different cross-section sizes was proposed.The existing BIS technology cannot accurately obtain the length and aging status of each cable segment along the line.The influence of the existence of local aging segments in a non-uniform aging cable on the accuracy of defect location were simulated and analyzed,and the source of error for defect location were further revealed based on the analysis of defect location principle of BIS,then a location correction coefficient M considering the influence of wave speed was proposed to improve the accuracy of defect localization in non-uniform aging cables.After introducing the coefficient M,the floating range of simulation defect localization deviation narrowed from-5.13%～5.13%to-0.44%～0.38%,and the experimental defect localization deviation was reduced from 2.19%to 0.39%.Then,an optimized calculation model was established to obtain the wave speed and length of each cable segment in the non-uniform aging cable,in which the location results of defects corrected by the wave velocity were introduced,and the effectiveness of the calculation model was further verified by simulations and experiments.On this basis,combined with the aging diagnosis criteria established,a non-destructive method was proposed to quantitatively diagnose the insulation aging status of each aging cable segment in non-uniform aging XLPE cable along the line.In addition,the overall quantitative evaluation scheme of the non-uniform aging state of the cable was proposed,which can reflect the overall aging condition of the cable and highlight the influence of the local severe aging segments on the operation reliability of the cable.The core work of cable residual life assessment is the prediction of the aging rate of cable insulation,and most existing cable life assessment models rely on the temperature information to predict the aging rate.However,it is difficult to determine the long-term operation temperatures of XLPE distribution cables since they are affected by the line load,climate environment and other factors.Moreover,for nonuniform aging cables,and it is difficult to obtain the temperature of each cable segment along the cable line.The validity of time-temperature equivalence principle used to convert the variation trend of EAB of XLPE at different aging temperatures was demonstrated.A method was proposed to predict the insulation aging trend of each cable segment along the line based on the corresponding EAB and the actual running time,which could get rid of the dependence of traditional prediction methods on temperature information.The reference curve of cable insulation aging trend was constructed according to the change process of EAB of XLPE with aging time in the accelerated aging test of XLPE cable segments.Then the current diagnostic value EAB of each cable segment was translated to the aging trend reference curve one by one to obtain the corresponding equivalent running time under the accelerated aging condition,and the corresponding "conversion factor" for each cable segment could be further calculated by the ratio of the actual running time and the equivalent running time obtained,which could be further used to extrapolate the aging trend reference curve to predict the insulation aging trend of each cable segment.On this basis,the life end criterion for cable insulation was set when the retention rate of EAB of XLPE is 50%,and then a residual life assessment model for cable segments along the cable line was constructed.Finally,the effectiveness of the model proposed was verified by experiments. |
PDF Full Text Request